Health campaigners have warned that attempts to use the London 2012 Olympics to improve public health may be undermined by today's announcement that one in five meals served to fans at the games will come from McDonald's.

Launching their food strategy, games organisers said that 3m of the 14m meals served during the two-week event will be prepared by the fast food chain. All branded soft drinks will be provided by Coca-Cola and the only other branded food on sale will be Cadbury's chocolate. McDonald's and Coca-Cola both sponsor the International Olympic Committee, which passes several million pounds of those revenues to London's organisers, while Cadbury pays £20m as a direct sponsor of the 2012 games.

The British Heart Foundation (BHF) and the National Obesity Forum (NOF) said the policy undermined claims from Lord Coe, the chairman of the London organising committee, and Tessa Jowell, the Olympics minister, that one of the main legacies of hosting the Olympics will be improved health across the nation. McDonald's outlets will be erected across the athletes' village, the main Olympic park and in the media centre.

"Health does not seem to be high on their agenda," said Ruairi O'Connor, head of policy at the BHF. "The focus on fast food, where the primary products are high in fat, sugar and salt, means we question whether the health legacy which has been promised will be fulfilled."

Dr David Haslam, chairman of the NOF, said: "The government is going to have to be very creative in how it promotes this relationship, which aggressively promotes McDonald's and Coca-Cola, as improving the health of the nation.

"The government is saying that hosting the Olympics will improve health standards but on the other hand is offering large amounts of Big Macs and Coke to supporters. Is this cynical advertising of inappropriate food, or will these companies embrace the spirit of the games and promote their healthier options over and above the rest?"

During the bidding, Coe claimed winning the 2012 games would help in the fight against childhood obesity.

"People have said don't waste money on the Olympics, put it into fighting obesity, making people healthier," he said. "I say the best way to do all that is to stage the Olympics here in 2012."

The announcement of the extent of McDonald's role, came as part of a food strategy that otherwise aims to promote local and seasonal foods across 40 Olympic venues in what has been dubbed the biggest peace-time catering operation in the world. A spokeswoman for the London organising committee said there will be more choice of food than at any other sporting venue in the country and stressed that food must be procured in accordance with a code on sustainable sourcing. All bananas, chocolate, tea, coffee and sugar will be certified as Fairtrade, while all meat, poultry and eggs will be sourced only from Britain. Drinking water will be freely available.

"McDonald's has a great reputation around how it sources its food," she said. "All McDonald's milk is organic, for example, and its eggs are free range. They will also provide healthy choices on their menus."

McDonald's said it expected its outlets to offer the same range as high street branches, including salads and porridge as well as burgers and chips. "We pay money to have the exclusive worldwide rights on branded food provision at the Olympics," said a spokeswoman. "Without commercial sponsors like McDonald's the games simply wouldn't happen."

Coca-Cola said it was committed to providing a variety of drinks including still, sparkling, low- and no-sugar drinks, pure fruit juice, water, and energy drinks.